Rhenium-free nickel base superalloy of low density

ABSTRACT

The invention relates to a substantially rhenium-free nickel base alloy showing a high creep resistance and relatively low density which comprises in % by weight: aluminum from 3.0 to 7.7, cobalt from 0 to 16.8, chromium from 3 to 11.8, molybendum from 3.1 to 11.3, tantalum from 0 to 3.9. In addition to nickel and unavoidable impurities this alloy may further comprise one or more of titanium, tungsten, carbon, phosphorus, copper, zirconium, silicon, hafnium, yttrium, niobium, and germanium.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 of European Patent Application No. 15166317.6, filed May 5, 2015, the entire disclosure of which is expressly incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nickel base alloy which is substantially free of rhenium but at the same time shows the creep resistance of a nickel base superalloy of the second generation and a density which is lower than that of comparable alloys.

2. Discussion of Background Information

In gas turbines such as stationary gas turbines of aircraft engines nickel base superalloys are employed, for example, as blade materials because these materials exhibit a sufficient strength for high mechanical stress even at high operating temperatures. For example, turbine blades of stationary gas turbines or jet engines are exposed to exhaust gas temperatures of up to 1500° C. and at the same time are subject to high mechanical stress caused by centrifugal forces. Under these conditions it is important in particular for the creep resistance of the material employed to meet the corresponding requirements. To further increase the creep resistance it has also been practice for decades to manufacture monocrystalline turbine blades in order to improve the creep resistance by avoiding grain boundaries.

The currently employed nickel base superalloys of the so-called second and third usually contain the element rhenium at a concentration of from three to six percent because rhenium further improves the creep resistance.

Due to the relative scarcity of rhenium it is however, very expensive to include rhenium in an alloy. In view thereof, it has already been proposed to reduce the proportion of rhenium or to exclude rhenium altogether but to at the same time retain the mechanical properties of the alloy, in particular with respect to creep resistance. Studies in this regard exist from A. Heckl, S. Neumeier, M. Oaken, R. F. Singer, The effect of Re and Ru on γ/γ′microstructure, γ-solid solution strengthening and creep strength in nickel-base superalloys”, in Material Science and Engineering A 528 (2011) 3435-3444 and Paul J, Fink, Joshua L. Miller, Douglas G. Konitzer, “Rhenium Reduction—Alloy Design Using an Economically Strategic Element”, JOM, 62(2010), 55-57. In addition, corresponding alloys are also subject matter of patents and patent applications, for example, EP 2 725 110 A1, DE 102010037046, US 2011/0076180 A1, EP 2 314 727 A1, EP 2 305 847 A1, EP 2 305 848 A1, US 2013/0129522 A1, WO 2013/083101 A1, EP 2 576 853 B1, WO 2009/032578 A1, WO 2009/032579 A1, EP 0 962 542 A1, U.S. Pat. No. 6,054,096, US 2013/0230405 A1 and US 2010/0135846 A1. The entire disclosures of these documents are incorporated by reference herein.

For instance, EP 2 725 110 A1 discloses a nickel base superalloy which is substantially free of rhenium and exhibits a solidus temperature of higher than 1320° C., where at temperatures from 1050° C. to 1100° C. precipitates of a γ′-phase in a γ-matrix are present in a proportion of from 40 to 50 vol. %, the γ/γ′-mismatch at temperatures from 1050° C. to 1100° C. ranges from −0,15% to −0,25%, and the concentration of tungsten in the γ-matrix is higher than that in the precipitated γ′-phases. The alloy has the following composition: aluminum from 11 to 13 at.-%, cobalt from 4 to 14 at.−%, chromium from 6 to 12 at.-%, molybdenum from 0.1 to 2 at.-%, tantalum from 0.1 bis 3.5 at.-%, titanium from 0.1 bis 3.5 at.-%, tungsten from 0.1 to 3 at.-%, remainder nickel and unavoidable impurities.

Although proposals for the reduction of rhenium or for rhenium-free nickel base superalloys thus already exist, there still is a desire to have available rhenium-reduced or rhenium-free nickel base superalloys whose mechanical properties and in particular, properties at high temperature such as creep resistance are comparable to those of the currently employed rhenium-free and rhenium containing nickel base superalloys and which exhibit further improved properties such as, e.g., a lower density compared to these existing alloys and nickel base superalloys of the second and third generation. It would also be advantageous to have available a substantially rhenium-free nickel base superalloy which exhibits mechanical properties such as creep resistance which are comparable to those of currently employed nickel base superalloys of the second and third generation. It would further be advantageous for these alloys to have as low a density as possible and a good annealability, and to be capable of being produced economically and efficiently and in monocrystalline or directionally solidified form. Compared to the rhenium-free alloys disclosed in EP 2 725 110 A1 these alloys should exhibit, at a comparable creep resistance, improved properties, in particular a lower density, a lower proportion of residual eutectic and an improved annealability.

SUMMARY OF THE INVENTION

The present invention provides a nickel base alloy exhibiting high creep resistance and being substantially free of rhenium, wherein the alloy comprises the following elements in % by weight based on the total weight of the alloy:

aluminum from 3.0 to 7.7, e.g., from 3.4 to 7.7, from 3.8 to 7.7, from 4.1 to 7.7, from 4.7 to 7.7, or from 5.0 to 5.4;

cobalt from 0 to 16.8, e.g., from 2.6 to 13.6, or from 2.9 to 13.3;

chromium from 3 to 11.8, e.g., from 4 to 11.8, from 5 to 11.8, from 6 to 11.8, from 6.3 to 7.3, or from 6.6 to 7;

molybendum from 3.1 to 11.3, e.g., from 3.3 to 11.3, from 3.4 to 11.3, from 3.6 to 11.3, from 3.7 to 4.7, or from 4 to 4.4; and

tantalum from 0 to 3.9, e.g., from 0 to 0.5, or from 0 to 0.2.

In addition to nickel (which is usually present in a concentration of at least 67 at-%) and unavoidable impurities this alloy may further comprise one or more (e.g. at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, or at least 8) other elements, for example one or more of the following (in % by weight):

titanium (e.g., up to 6, up to 5, up to 4, up to 3.6, from 2.8 to 3.6, or from 3.1 to 3.5),

tungsten (e.g., up to 11.3, from 7.4 to 8.4, or from 7.7 to 8.1),

carbon (e.g., up to 0.05),

phosphorus (e.g., up to 0.015),

copper (e.g., up to 0.05),

zirconium (e.g., up to 0,015),

silicon (e.g., up to 6.0, up to 5.0, up to 4.0, up to 3.0, or up to 2.0),

sulfur (e.g., up to 0.001),

iron (e.g., up to 0.15),

manganese (e.g., up to 0.05),

boron (e.g., up to 6.0, up to 5.0, or up to 4.0),

hafnium (e.g., up to 4.0, up to 3.0, up to 2.0, or up to 1.0),

yttrium (e.g., up to 0.002),

niobium (e.g., up to 8.0, up to 7.0, up to 6.0, up to 5.0, up to 4.0, up to 3.0, up to 2.0, or up to 1.0), and

germanium (e.g., up to 8.0, up to 7.0, up to 6.0, up to 5.0, up to 4.0, up to 3.0, up to 2.0, or up to 1.0).

In one aspect of the alloy of the present invention, the alloy may comprise less than 5% by weight cobalt, e.g., less than 4% by weight cobalt.

In another aspect of the alloy of the present invention, the alloy may comprise more than 11% by weight cobalt, e.g., more than 12%, or more than 13% of cobalt.

In a further aspect, the alloy may have a density of not higher than 8.5 g/cm³, e.g., not higher than 8.4 g/cm³ and/or the alloy may have a solidus temperature of higher than 1320° C. and/or a residual eutecticum of not more than 4%, e.g, not more than 3%.

The present invention also provides an article that is made of the alloy of the present invention as set forth above (including the various aspects thereof).

In one aspect of the article, the alloy may be monocrystalline or directionally solidified.

In another aspect of the article, the article may be a component of a gas turbine or an aircraft engine such as, e.g., a turbine blade.

The present invention also provides a method of making a nickel base alloy as set forth above (including the various aspects thereof). The method comprises melting together elements in proportions which result in the alloy of the present invention.

The present invention provides a nickel base superalloy which contains at least the elements Al, Cr and Mo (and often also Co) and which has been optimized with respect to the following:

-   -   as high as possible weight-based mixed crystal hardening index         I_(SSS) in the matrix;     -   optimum γ-morphology:         -   γ/γ′-mismatch at 1100° C. from -0.1 to −0.5%         -   γ′-proportion at 1100° C. from 44 to 48 mol-%     -   solidus temperature >1320° C.

Regarding the above, I_(SSS)=2.44x^(γ) _(Re)+1.22x^(γ) _(W)+x^(γ) _(M0) x^(γ) _(i)=concentration in at.-% of the respective elements in the matrix) and the γ/γ′-mismatch is defined as the normalized difference of the lattice constants of the two phases γ and γ′:

$\frac{a_{\gamma^{\prime}} - a_{\gamma}}{\frac{1}{2}*\left( {a_{\gamma^{\prime}} + a_{\gamma}} \right)}$

According to the above optimization, a nickel base alloy may have the following exemplary composition, in % by weight based on the total weight of the alloy: aluminum from 3.0 to 7.7, cobalt from 0 to 16.8, chromium from 3 to 11.8, molybdenum from 3.1 to 11.3, tantalum from 0 to 3.9, titanium from 0 to 6.0, tungsten from 0 to 11.3, carbon from 0 to 0.05, phosphorus from 0 to 0.015, copper from 0 to 0.05, zirconium from 0 to 0.015, silicon from 0 to 6.0, sulfur from 0 to 0.001, iron from 0 to 0.15, manganese from 0 to 0.05, boron from 0 to 6.0, hafnium from 0 to 4.0, yttrium from 0 to 0.002, niobium from 0 to 8.0, germanium from 0 to 8.0, remainder nickel and unavoidable impurities.

As can be seen, the alloy is substantially free of rhenium, i.e., it contains rhenium, if at all, only in trace amounts (e.g., not more than 0.001% by weight). The alloy may further also be substantially free of tantalum.

Further, a nickel base alloy of the present invention may have the following composition, in % by weight: aluminum from 4.1 to 7.7, cobalt from 0 to 16.8, chromium from 6 to 11.8, molybdenum from 3.6 to 11.3, tantalum from 0 to 3.9, titanium from 0 to 3.6, tungsten from 0 to 11.3, carbon from 0 to 0.05, phosphorus from 0 to 0.015, copper from 0 to 0.05, zirconium from 0 to 0.015, silicon from 0 to 0.01, sulfur from 0 to 0.001, iron from 0 to 0.15, manganese from 0 to 0.05, boron from 0 to 0.003, hafnium from 0 to 0.15, yttrium from 0 to 0.002, remainder nickel and unavoidable impurities.

Another exemplary embodiment the alloy of the present invention may have the following composition, in % by weight relative to the total weight of the alloy: aluminum from 4.4 to 5.7, cobalt from 2.6 to 13.6, chromium from 6.3 to 7.3, molybdenum from 3.7 to 4.7, tantalum from 0 to 0.5, titanium from 2.8 to 3.6, tungsten from 7.4 to 8.4, carbon from 0 to 0.05, phosphorus from 0 to 0.015, copper from 0 to 0.05, zirconium from 0 to 0.015, silicon from 0 to 0.01, sulfur from 0 to 0.001, iron from 0 to 0.15, manganese from 0 to 0.05, boron from 0 to 0.003, hafnium from 0 to 0.15, yttrium from 0 to 0.002, remainder nickel and unavoidable impurities.

Yet another exemplary embodiment the alloy of the present invention may have the following composition, in % by weight relative to the total weight of the alloy: aluminum from 5.0 to 5.4, cobalt from 2.9 to 13.3, chromium from 6.6 to 7, molybdenum from 4 to 4.4, tantalum from 0 to 0.2, titanium from 3.1 to 3.5, tungsten from 7.7 to 8.1, carbon from 0 to 0.05, phosphorus from 0 to 0.015, copper from 0 to 0.05, zirconium from 0 to 0.015, silicon from 0 to 0.01, sulfur from 0 to 0.001, iron from 0 to 0.15, manganese from 0 to 0.05, boron from 0 to 0.003, hafnium from 0 to 0.15, yttrium from 0 to 0.002, remainder nickel and unavoidable impurities.

In a further embodiment of the nickel base alloy of the present invention the alloy comprises less than 5% by weight, e.g., less than 4% by weight, of cobalt. Since cobalt has a higher atomic weight than nickel, a relatively low cobalt concentration has a favorable effect on the total density of the nickel base alloy, and thus on the total weight of the target component made from the alloy.

However, alternatively the nickel base alloy of the present invention may comprise more than 11% by weight, e.g., more than 13% by weight, of cobalt. A corresponding cobalt concentration has a positive effect on segregation during solidification and the stability of the microstructure with respect to undesired formation of TCP phases.

It also is preferred for the nickel base alloy of the present invention to comprise at least 67 at.-%, for example at least 68 at.-% nickel.

The nickel base alloy of the present invention may further exhibit one or more (for example, all) of the following properties:

-   -   a density of not higher than 8.5 g/cm³, preferably not higher         than 8.4 g/cm³;     -   a solidus temperature higher than 1320° C.;     -   44 to 48 vol.-% of precipitates of a γ′-phase in a γ-matrix at a         temperature of 1100° C.;     -   a γ/γ′-mismatch ranging from −0.1% to −0.5% at a temperature of         1100° C.;     -   a residual eutectic of not more than 4%, e.g., not more than 3%.

The term “unavoidable impurities” in the alloy as used herein and the appended claims means elements whose presence in the alloy is unintentional but cannot be avoided for technical reasons or can only be avoided with extreme difficulty. For example, the following elements may be present in the alloy of the present invention in the form of trace elements, in % by weight: Bi up to 0.00003, Se up to 0.0001, Tl up to 0.00005, Pb up to 0,0005, and Te up to 0.0001.

The alloy of the present invention can be used, for example, in the manufacture of components of gas turbines, preferably turbine blades, and the like, which components may be present in monocrystalline or directionally solidified form.

BRIEF DESCRIPTION OF THE DRAWING

The attached FIGURE shows a Larson-Miller plot for illustrating the creep resistance of the alloy of the present invention compared to that of known alloys.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description in combination with the drawings making apparent to those of skill in the art how the several forms of the present invention may be embodied in practice.

EXAMPLE

An alloy according to the present invention whose composition can be taken from the following table was prepared (Alloy 1). Alloys 2 and 3 were chosen as comparative alloys, Alloy 3 corresponding in its chemical composition essentially to that of the rhenium containing material CMSX-4 and Alloy 2 being the rhenium-free nickel base superalloy disclosed in EP 2 725 110 A1. The components of the alloys are indicated in wt. % (remainder Ni and unavoidable impurities).

Alloy No. Al Co Cr Mo Re Ta Ti W 1 5.2 3.1 6.8 4.2 — — 3.3 7.9 2 4.8 8.6 5.0 1.4 — 10.1 1.3 8.8 3 5.6 9.0 6.5 0.6 3.0 6.5 1.0 6.0

Alloy 1 according to the present invention was prepared in a Labor-Bridgman casting apparatus in three-bar geometry. Each of the bars had a diameter of 12 mm and a length of 180 mm and exhibited a typical dendritic microstructure with a dendrite distance of about 230 μm. The proportion of residual eutectic of 2.8% is very low (Alloy 2 and Alloy 3 showed a residual eutectic of 6.5% and 9.0% respectively). If suitably heat-treated (see below), Alloy 1 has a typical, completely cubic y′-phase morphology.

Additionally, push-pull experiments were conducted on cylinders (diameter 4.0 mm, height 6.4 mm) made from the completely heat-treated Alloy 1 to 3. The front faces of the cylinders were finished to ensure they were parallel. All creep experiments were conducted at constant stress with the following parameters: 1100° C./1137 MPa, 1050° C./200 MPa, 950° C./1300 MPa, 950° C./1400 MPa. Die corresponding creep curves are shown in the FIGURE (1% plastic deformation, DB material, λ=220 μm).

As can be seen from the FIGURE, Alloy 1 according to the invention (L1) shows a creep resistance which is substantially the same as that of the rhenium-free Alloy 2 (L2), the creep resistances of these alloys being similar to the creep resistance of Alloy 3 which corresponds to a nickel base superalloy of the second generation. However, compared to Alloy 1 and Alloy 2, Alloy 1 particularly shows a lower density Analysis of the microstructure of Alloy 1 according to the invention after creep does not reveal any TCP phase formation.

This shows that the present invention can provide nickel base alloys which do not depend on the presence of the not readily available element rhenium but nevertheless can exhibit high temperature mechanical properties such as creep resistance similar to those of known rhenium-containing alloys and additionally have a lower density than known rhenium-containing and rhenium-free alloys.

In the following table the properties of Alloys 1-3 are compared to each other.

Alloy 1 Alloy 3 as Alloy 1 Alloy 2 as Property calculated as measured as measured measured Density, g/cm³ 8.3   8.4   9.0 8.7 Cost, Euro/kg 81  103 207 As of 2012 Liquidus temp., ° C. 1373 1371 1371 1381 Solidus temp., ° C. 1348 1302* 1316* 1338 γ′-Solvus temp. ° C. 1232 1255 1242 1257 γ/γ′-Mismatch, −0.5  −0.45**  −0.02** −0.17 1100° C., % γ′ Proportion, 44.0 — — 44.9 1100° C., mol-% *as cast **room temperature

To be particularly emphasized is the relatively low density of Alloy 1 according to the invention. Further, the raw material costs as of 2012 are only about 40% of those for Alloy 3 (CMSX-4). The γ/γ′-mismatch values could be measured only at room temperature; usually the values are higher at higher temperatures.

Annealing of Alloy 1 may for example be carried out in two stages as follows:

-   -   Heating the alloy at 4 K/min up to 1285° C.,     -   Holding for 2 h at 1285° C.,     -   Heating the alloy at 1 K/min up to 1300° C.;     -   Subsequent rapid cooling.

Additionally, following solution annealing, Alloy 1 may be subjected to one or both of the following precipitation hardening treatments:

Precipitation hardening treatment 1:

Temperature Heating rate Hold time 1000° C. 4 K/min 1050° C. 1 K/min 1050° C. 1 h 20° C. Rapid cooling

Precipitation hardening treatment 2:

Temperature Heating rate Hold time 840° C. 4 K/min 870° C. 1 K/min 870° C. 24 h  20° C. Rapid cooling

Annealing times of more than 2 hours at 1050° C. or higher temperatures result in an excessive aging of the microstructure.

In summary, it can be stated that the above-described alloy according to the invention shows the following properties in particular:

Creep resistance close to that of CSMX-4

Low density of 8.4 g/cm³ (comparison: CSM X-4: 8.7 g/cm³)

Low residual eutectic of 2.8% (comparison: CSMX-4: 9.0%)

Good annealability (holding for 8.5 hours at 1285° C./1300° C.).

Low tendency for TCP phase formation

Very low cost compared to CSMX-4.

It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. 

What is claimed is:
 1. A nickel base alloy exhibiting high creep resistance and being substantially free of rhenium, wherein the alloy comprises the following elements in % by weight relative to the total weight of the alloy: aluminum from 3.0 to 7.7 cobalt from 0 to 16.8 chromium from 3 to 11.8 molybendum from 3.1 to 11.3 tantalum from 0 to 3.9.
 2. The alloy of claim 1, wherein the alloy comprises: aluminum from 3.4 to 7.7 cobalt from 0 to 16.8 chromium from 4 to 11.8 molybendum from 3.3 to 11.3 tantalum from 0 to 3.9.
 3. The alloy of claim 1, wherein the alloy comprises: aluminum from 3.8 to 7.7 cobalt from 0 to 16.8 chromium from 5 to 11.8 molybendum from 3.4 to 11.3 tantalum from 0 to 3.9.
 4. The alloy of claim 1, wherein the alloy comprises: aluminum from 4.1 to 7.7 cobalt from 0 to 16.8 chromium from 6 to 11.8 molybendum from 3.6 to 11.3 tantalum from 0 to 3.9.
 5. The alloy of claim 1, wherein the alloy comprises: aluminum from 4.7 to 7.7 cobalt from 2.6 to 13.6 chromium from 6.3 to 7.3 molybendum from 3.7 to 4.7 tantalum from 0 to 0.5 titanium from 2.8 to 3.6 tungsten from 7.4 to 8.4.
 6. The alloy of claim 1, wherein the alloy comprises: aluminum from 5.0 to 5.4 cobalt from 2.9 to 13.3 chromium from 6.6 to 7 molybendum from 4 to 4.4 tantalum from 0 to 0.2 titanium from 3.1 to 3.5 tungsten from 7.7 to 8.1.
 7. The alloy of claim 1, wherein the alloy further comprises: titanium from 0 to 6.0 tungsten from 0 to 11.3 carbon from 0 to 0.05 phosphorus from 0 to 0.015 copper from 0 to 0.05 zirconium from 0 to 0.015 silicon from 0 to 6.0 sulfur from 0 to 0.001 iron from 0 to 0.15 manganese from 0 to 0.05 boron from 0 to 6.0 hafnium from 0 to 4.0 yttrium from 0 to 0.002 niobium from 0 to 8.0 germanium from 0 to 8.0, remainder nickel and unavoidable impurities.
 8. The alloy of claim 7, wherein the alloy comprises: titanium from 0 to 5 silicon from 0 to 5.0 boron from 0 to 5.0 hafnium from 0 to 3.0 niobium from 0 to 6.0 germanium from 0 to 6.0.
 9. The alloy of claim 8, wherein the alloy comprises: titanium from 0 to 4 silicon from 0 to 4.0 boron from 0 to 4.0 hafnium from 0 to 3.0 niobium from 0 to 4.0 germanium from 0 to 4.0.
 10. The alloy of claim 9, wherein the alloy comprises: titanium from 0 to 4 silicon from 0 to 4.0 boron from 0 to 4.0 hafnium from 0 to 3.0 niobium from 0 to 4.0 germanium from 0 to 4.0.
 11. The alloy of claim 10, wherein the alloy comprises: titanium from 0 to 3.6 silicon from 0 to 2.0 boron from 0 to 2.0 hafnium from 0 to 1.0 niobium from 0 to 1.0. germanium from 0 to 1.0.
 12. The alloy of claim 1, wherein the alloy comprises less than 5% by weight cobalt.
 13. The alloy of claim 1, wherein the alloy comprises more than 11% by weight cobalt.
 14. The alloy of claim 1, wherein the alloy has a density of not higher than 8.5 g/cm³.
 15. The alloy of claim 1, wherein the alloy has a solidus temperature of higher than 1320° C.
 16. The alloy of claim 1, wherein the alloy has a residual eutecticum of not more than 4%.
 17. An article made of the alloy of claim
 1. 18. The article of claim 17, wherein the article is monocrystalline or directionally solidified.
 19. The article of claim 21, wherein the article is a component of a gas turbine or an aircraft engine.
 20. A method of making a nickel base alloy, wherein the method comprises melting together metals in proportions which result in the alloy of claim
 1. 